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Use the bond-dissociation enthalpies in Table 4-2 (page

Organic Chemistry | 8th Edition | ISBN: 9780321768414 | Authors: L.G. Wade Jr ISBN: 9780321768414 33

Solution for problem 20P Chapter 4

Organic Chemistry | 8th Edition

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Organic Chemistry | 8th Edition | ISBN: 9780321768414 | Authors: L.G. Wade Jr

Organic Chemistry | 8th Edition

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Problem 20P

Use the bond-dissociation enthalpies in Table 4-2 (page 143) to calculate the heats of reaction for the two possible first propagation steps in the chlorination of isobutane. Use this information to draw a reaction-energy diagram like Figure 4-8, comparing the activation energies for formation of the two radicals.

Step-by-Step Solution:

Solution 20P:

Here, we are going to calculate the heats of reaction for the two possible  propagation steps in the chlorination of isobutane.

Step 1.

The propagation steps in the chlorination of isobutane is as follows:

Formation of tertiary radical  or abstraction of 3o Hydrogen

Here,

Energy required to break the (CH3)3C- H bond    +381 kJ/mol (+91 kcal/mol)

Energy required to form the H-Cl  bond              -431 kJ/mol (-103 kcal/mol)

Total energy for reaction at the 3o position is Ho =  -50 kJ/mol (-12 kcal/mol)

Step 2.

Formation of primary radical  or abstraction of 1o Hydrogen

Here,

Energy required to break the (CH3)2CH-CH2-H bond    +410 kJ/mol (+98 kcal/mol)

Energy required to form the H-Cl  bond              -431 kJ/mol (-103 kcal/mol)

Total energy for reaction at the 1o position is Ho =  -21.0 kJ/mol (-5.0 kcal/mol)

Step 3 of 3

Chapter 4, Problem 20P is Solved
Textbook: Organic Chemistry
Edition: 8
Author: L.G. Wade Jr
ISBN: 9780321768414

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Use the bond-dissociation enthalpies in Table 4-2 (page

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